remote sensing

Article New Geodetic and Gravimetric Maps to Infer Geodynamics of Antarctica with Insights on Victoria Land

Antonio Zanutta 1,*, Monia Negusini 2, Luca Vittuari 1 , Leonardo Martelli 1, Paola Cianfarra 3 , Francesco Salvini 3, Francesco Mancini 4 , Paolo Sterzai 5, Marco Dubbini 6 and Alessandro Capra 4

1 Dipartimento di Ingegneria Civile, Chimica, Ambientale e dei Materiali—Università di Bologna, 40136 Bologna, Italy; [email protected] (L.V.); [email protected] (L.M.) 2 Istituto di Radioastronomia—Istituto Nazionale di Astrofisica, 40129 Bologna, Italy; [email protected] 3 Dipartimento di Scienze Geologiche, Università degli Studi Roma Tre, 00146 Roma, Italy; [email protected] (P.C.); [email protected] (F.S.) 4 Dipartimento di Ingegneria Enzo Ferrari—Università degli Studi di Modena e Reggio Emilia, 41125 Modena, Italy; [email protected] (F.M.); [email protected] (A.C.) 5 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale—OGS—“Infrastructures” Section (IRI), 34010 Sgonico (Trieste), Italy; [email protected] 6 Dipartimento di Storia Culture Civiltà—Università di Bologna, 40124 Bologna, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-051-2093111



Received: 30 July 2018; Accepted: 6 October 2018; Published: 10 October 2018


Abstract: In order to make inferences on the geodynamics of Antarctica, geodetic and gravimetric maps derived from past and new observations can be used. This paper provides new insights into the geodynamics of Antarctica by integrating data at regional and continental scales. In particular, signatures of geodynamic activity at a regional extent have been investigated in Victoria Land (VL, Antarctica) by means of Global Navigation Satellite System (GNSS) permanent station observations, data from the VLNDEF (Victoria Land Network for Deformation control) discontinuous network, and gravity station measurements. At the continental scale, episodic GNSS observations on VLNDEF sites collected for 20 years, together with continuous data from the International GNSS Service (IGS) and Polar Earth Observing Network (POLENET) sites, were processed, and the Euler pole position assessed with the angular velocity of the Antarctic plate. Both the Bouguer and the free-air gravity anomaly maps were obtained by integrating the available open-access geophysics dataset, and a compilation of 180 gravity measurements collected in the VL within the Italian National Program for Antarctic Research (PNRA) activities. As a result, new evidence has been detected at regional and continental scale. The main absolute motion of VL is towards SE (Ve 9.9 ± 0.26 mm/yr, Vn −11.9 ± 0.27 mm/yr) with a pattern similar to the transforms of the Tasman and Balleny fracture zones produced as consequence of Southern Ocean spreading. Residual velocities of the GNSS stations located in VL confirm the active role of the two main tectonic lineaments of the region, the Rennick–Aviator and the Lillie–Tucker faults with right-lateral sense of shear. The resulting VL gravity anomalies show a NW region characterized by small sized Bouguer anomaly with high uplift rates associated and a SE region with low values of Bouguer anomaly and general subsidence phenomena. The East and West Antarctica are characterized by a different thickness of the Earth’s crust, and the relative velocities obtained by the observed GNSS data confirm that movements between the two regions are negligible. In East Antarctica, the roots of the main subglacial highlands, Gamburtsev Mts and Dronning Maud Land, are present. The Northern Victoria Land (NVL) is characterized by more scattered anomalies. These confirm the differences between the Glacial Isostatic Adjustment (GIA) modeled and observed uplift rates that could be related to deep-seated, regional scale structures.

Remote Sens. 2018, 10, 1608; doi:10.3390/rs10101608 www.mdpi.com/journal/remotesensing Remote Sens. 2018, 10, 1608 2 of 27

Keywords: VLNDEF; gravity anomaly; GNSS time series; neo-tectonics; Antarctica geodynamics; crustal deformations; PNRA

1. Introduction In geosciences, GNSS (Global Navigation Satellite Systems) and gravimetric measurements are key methods to highlight geodynamic and geophysical phenomena of a geological region, and are a fundamental support in the formulation of deformation models at regional and continental scale, referring also to the interaction mechanisms of interaction between the solid Earth and the cryosphere. The joint analysis of residual geodetic and gravimetric signatures in Antarctica could support theories on the structure and the kinematic of the Earth’s lithosphere, characterizing the crustal thickness and the tectonic processes. Furthermore, it could provide evidence of active geodynamic and deformation processes that change the shape of the continents. However, the observed solid Earth deformation includes several signals produced by different geophysical phenomena, which are not easy to separate, being related to phenomena that act in a very different spatial and temporal scale or due to bias in modeling or limitations in the a priori hypothesis. Difficulties in the analysis of gravimetric and geodetic data could also arise from gaps and inhomogeneity in the spatial distribution and measurement accuracy (e.g., gravimetric maps based on data collected after terrestrial and airborne gravimetric campaigns across the last two decades). Many continuous GNSS stations were installed in the last two decades in Antarctica by national and international projects for different purposes. Some of them were included in the IGS (International GNSS Service) network and used in the ITRF (International Terrestrial Reference Frame) computation. Several authors processed subsets of such stations within studies related to geodynamics, glaciology and Glacial Isostatic Adjustment (GIA) modeling [1–6], but the data processing at continental scale of the full GNSS dataset has not been performed yet. The drawback of using permanent GNSS stations in the detection of surface motions could be attributed to the limited number and density of stations whenever a detailed picture of regional deformation phenomena is sought to support geodynamic modeling. A couple of projects oriented to the detailed investigation of the regional deformation field by dense GNSS measurements were implemented by the United States and Italian projects which are, respectively, called TAMDEF (Trans Antarctic Mountains Deformation) and VLNDEF (Victoria Land Network for Deformation control) [7–10]. The mapping of Earth’s gravity field and crustal thickness in Antarctica has been improved by the CHAMP (Challenging Minisatellite Payload) [11], GRACE (Gravity Recovery and Climate Experiment) and GOCE (Gravity field and steady state Ocean Circulation Explorer) satellite gravity missions [12]. Such dedicated gravity satellite missions provided data with a full coverage and consistent accuracy at spatial resolution of the order of 100 km [12–14], but a “polar gap” South of −83.3◦ latitude still exists due to satellite orbit inclination [12]. Towards the definition of a refined gravity field over the whole Antarctic continent, the recent use of Bedmap2 [15] to better define the gravity effects from topographic mass modeling must be mentioned [12]. Over the last two decades, airborne gravimetric campaigns have improved the definition of the gravity field in confined regions (see papers [16,17], among others). In order to obtain a better spatial resolution in the gravity field mapping with delineation of crustal features, terrestrial gravity compilation should be used [17]. Unfortunately, due to logistic difficulties in terrestrial gravimetric surveys, the Antarctic territories suffer from a lack of gravity data distribution and improvement in airborne gravimetric surveys does not allow the full spatial coverage of gravimetric measurements. A first compilation of continental scale gravity anomaly grid for Antarctica was provided by Scheinert [18], on the initiative of the International Association of Geodesy (IAG) Sub-Commission 2.4f “Gravity and Geoid in Antarctica” (AntGG), as a collection and integration of 13 million gravity data points from terrestrial, airborne and shipborne surveys acquired by the international geosciences community [18]. The authors carefully considered a large Remote Sens. 2018, 10, 1608 3 of 27 Remote Sens. 2018, 10, x FOR PEER REVIEW 3 of 27 heterogeneity within the gravity dataset during the processing and provided the grid of Free-Air processing and provided the grid of Free-Air Gravity (FAG) and Bouguer Gravity (BG) anomalies Gravity (FAG) and Bouguer Gravity (BG) anomalies with a grid spacing of 10 km. By using this with a grid spacing of 10 km. By using this dataset, shorter wavelength features (for instance, dataset, shorter wavelength features (for instance, intra-crustal density variations) could be more intra-crustal density variations) could be more easily resolved. easily resolved. Several international and national projects in Antarctica have contributed to model geodynamic Several international and national projects in Antarctica have contributed to model geodynamic scenarios from the cross interpretation of geodetic and geophysical data at regional and continental scenarios from the cross interpretation of geodetic and geophysical data at regional and continental scale [19,20]. These activities are coordinated by the SCAR (Scientific Committee on Antarctic scale [19,20]. These activities are coordinated by the SCAR (Scientific Committee on Antarctic Research) Research) through several initiatives such as the SERCE (Solid Earth Cryosphere Response and through several initiatives such as the SERCE (Solid Earth Cryosphere Response and Evolution) Evolution) program, which focuses on the integration of geological, geodetic and geophysical program, which focuses on the integration of geological, geodetic and geophysical measurements in measurements in both the GIA models and the dynamic behavior of the polar cap. both theThis GIA paper models focuses and theon dynamic updating behavior GNSS ofand the polargravimetric cap. measurements at the Antarctic continentalThis paper scale focuses and Victoria on updating Land GNSS(VL) regional and gravimetric scale (Figure measurements 1a,b) with athistorical the Antarctic and new continental data. At scalethe continental and Victoria scale, Land (VL)historical regional gravimetric scale (Figure data1a,b) collected with historical by shipborne and new gravimetry data. At the were continental merged scale,with data historical provided gravimetric by the AntGG data collected project. by Moreover shipborne, the gravimetry processing were of the merged huge withdataset data collected provided by bycontinuous the AntGG GNSS project. stations Moreover, since their the processing installation of was the hugeperformed dataset to collected define the by continental continuous surface GNSS stationsdeformation since theirpattern. installation With respect was performed to the regional to define extent, the continental this paper surface introduces deformation new terrestrial pattern. Withgravimetric respect tomeasurements the regional extent,carried this out paper in North introduces Victoria new Land terrestrial (NVL, gravimetric East Antarctica), measurements where carriedsignificant out anomalies in North in Victoria the deformation Land (NVL, field East from Antarctica), GNSS measurements where significant were found. anomalies In particular, in the deformationsigns of displacement field from due GNSS to measurementstectonic were found were found. in the Insouthern particular, Rennick signs glacier of displacement area [20,21] due and to tectonicneotectonics were was found inferred in the southernin the continent Rennick interior glaciers area [22]. [20 The,21 ]VL and tectonic neotectonics framework was inferred (Figure in1b) the is continentcharacterized interiors by the [22 presence]. The VL of tectonic a series framework of NW-SE (Figureregional1 b)faults is characterized with a main byright-lateral the presence sense of aof seriesshear ofsince NW-SE Upper regional Cenozoic faults withtimes a main[23]. right-lateralThis activity sense relates of shear to the since on-land Upper Cenozoicpropagation times of [ 23the]. ThisSouthern activity Ocean relates fracture to the zones on-land [24]. propagation of the Southern Ocean fracture zones [24].

FigureFigure 1.1.( a()a) Main Main physiographic physiographic features features of theof the Antarctica Antarctica continent continent from from Bedmap-2 Bedmap-2 dataset dataset [15] and[15] (band) simplified (b) simplified tectonic tectonic setting setting of Victoria of Vict Landoria Land (VL), (VL), modified modified from [from20]. [20].

GravimetricGravimetric data data currently currently available available are are poorly poorly distributed distributed or absentor absent within within NVL NVL [25] [25] and and do not do supportnot support a reliable a reliable mapping mapping of gravimetric of gravimetric anomalies. anomalies. To cover To this cover gap, newthis datagap, will new be data provided will inbe thisprovided paper in this order paper to support in order the to interpretation support the interpretation of alleged neotectonic of alleged phenomena. neotectonic phenomena.

2. Data Availability The dataset is a compilation of daily observations from 208 GNSS stations worldwide, 100 of which are located in Antarctica and collected from 1998 to 2017 (see also Supplementary Materials,

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2. Data Availability The dataset is a compilation of daily observations from 208 GNSS stations worldwide, 100 of whichRemote are locatedSens. 2018, in10, Antarcticax FOR PEER REVIEW and collected from 1998 to 2017 (see also Supplementary4 Materials, of 27 Tables S1–S3). GNSS data collected by the VLNDEF project were also included to provide insights aboutTables geodynamic S1–S3). GNSS phenomena data collected within bythe the NVLVLNDEF area project (Figures were2 andalso 3included; Table1 to). provide The gravimetric insights datasetabout used geodynamic in this paper phenomena is from terrestrial within the and NVL satellite area (Figures gravimetric 2 and data 3; acquiredTable 1). The by the gravimetric international geosciencesdataset communityused in this [ 12paper], in additionis from terrestrial to new data and provided satellite bygravimetric researchers data involved acquired in by the the PNRA (Italianinternational National Program geosciences for community Antarctic Research)[12], in addition scientific to new activities data provided (Figure by2 ).researchers involved in the PNRA (Italian National Program for Antarctic Research) scientific activities (Figure 2).

FigureFigure 2. Overview 2. Overview over over the the location location of of Global Global NavigationNavigation Satellite Satellite System System (GNSS) (GNSS) stations stations and and gravitygravity sites sites used used in the in the present present work. work. The The top-left top-left inset shows shows locations locations of ofterrestrial terrestrial gravimetric gravimetric measurements.measurements. The The cyan cyan color color defines defines the the area area wherewhere British Antarctic Antarctic Survey Survey (BAS) (BAS) airborne airborne data data are availableare available [25]; [25]; the the yellow yellow color color identifies identifies anan areaarea covered with with shipborne shipborne gravimetric gravimetric data data[20]; [20]; white color defines the areas not covered by airborne and shipborne data. The top-right inset depicts white color defines the areas not covered by airborne and shipborne data. The top-right inset depicts the Victoria Land Network for Deformation control (VLNDEF) network, surveyed in the frame of the the Victoria Land Network for Deformation control (VLNDEF) network, surveyed in the frame of the Italian National Program for Antarctic Research (PNRA) over 20 years. Italian National Program for Antarctic Research (PNRA) over 20 years.

Remote Sens. 2018, 10, 1608 5 of 27 Remote Sens. 2018, 10, x FOR PEER REVIEW 5 of 27

Figure 3. LocationsLocations of of the the 208 208 GNSS GNSS stations stations distributed distributed worldwide worldwide and and providing providing data data under under the supervisionthe supervision of different of different projects projects and institutions: and institutions: VLNDEF VLNDEF (ftp://ftp.ira.inaf.it/pub/ogia/), (ftp://ftp.ira.inaf.it/pub/ogia/ Polar Earth), ObservingPolar Earth Network Observing (POLENET) Network (ftp://data-out.unavc (POLENET) (ftp://data-out.unavco.org/pub/rinex/obs/o.org/pub/rinex/obs/) and International GNSS) and ServiceInternational (IGS) (ftp://igs.ensg.ign.fr/p GNSS Service (IGS) (ftp://igs.ensg.ign.fr/pub/igs/data/ub/igs/data/); Supplementary Materials); Supplementary available, Tables Materials S1–S3); yellowavailable, triangles Tables identify S1–S3); yellowIGS stations triangles adopted identify to de IGSfine stations the geodetic adopted datum to define within the the geodetic processing datum of withinthe GNSS the data processing using Bernese of the GNSS GNSS data software using [26]. Bernese GNSS software [26].

Table 1. Occupation (days) of VLNDEFVLNDEF stations from 1998 to 2017.

IDID\yryr 98 98 99 99 00 00 01 01 02 02 03 03 04 04 05 05 06 06 07 08 08 09 09 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17 TNB1TNB1 39 39 181181 331 331 361 361 334 334 356 356 330 330 329 329 362 362 275 275 4444 18 18 208208 364 364 167 167 322 322 9090 84 84 365365 341 341 VL01VL01 - - 2 2 - - - - 18 18 88 88 63 63 116116 88 - 35 35 100100 104 104 4747 106106 136 136 109 109 365 365 366 366 365 365 VL02VL02 - -6 62 2 - - 1 1 19 19 - - 4 4 8 8 ------14 14 - -16 16 - - - - VL03 - 6 2 - - 16 - 3 19 - 10 - - - 12 - 33 - - - VL03 - 6 2 - - 16 - 3 19 - 10 - - - 12 - 33 - - - VL04 - - - - 3 16 - 3 2 - - - 16 - - - 17 - - - VL04VL05 - -- 4 - - - - 3 21 16 43 34- 263 372 - 49- -101 1689 -117 91 - 67- 17118 73- 65 - 76 - VL05VL06 - - 4 - - 1 - - 21 - 43 15 34 36 26 - 37 17 - 49 - 101 - 89 - 117 - 91 21 67 5 118 21 73 25 65 - 76 - VL06VL07 - -- 41 5- -- 615 15 36 - - 7 17 10 - - 7 - -- 15 - -21 15 5 - 21 33 25 - - - VL07VL08 - -4 -5 6 - - 6 -15 9 - - 7 - 10 13 - 7 9 - -15 - - -15 6 - -33 4 23- - - VL08VL09 - -- 66 2- -- -9 19 - -- - 13 13 - 9 5 ------6 9 - - 134 23 - - - VL10 - 4 4 - - 25 34 11 33 - 4 - - - - - 12 - - - VL09 - 6 2 - - 19 - - 13 - 5 - - - 9 - 13 - - - VL11 - - 4 - - 7 - - 17 - - - - 4 6 - 13 - - - VL10VL12 - -4 64 2- -- 525 52 34 55 11 26 33 36 - 4 10 ------7- -12 15 365- 366- 365- VL11VL13 - -- 24 7- -- -7 10 - -- - 17 13 - - 7 - -- 74 -6 13 - -13 3 29- - - VL12VL14 - -6 10 2 2- -5 -52 45 55 34 26 26 36 26 - 10 5 - -- 4- -7 15 - -15 10 365 - 366 - 365 - VL13VL15 - -2 -7 6 - - - -10 18 - - - - 13 7 - 7 - - 2 7 17 - 413 14 - - 3 - 29 26 - - VL14VL16 - -10 - 2 8 - - - -45 15 34 11 26 - 26 13 - 5 7 - 1 4 5 - 115 - - -10 20 - - - - VL17 - - 6 - - 27 46 - 13 - - 1 - 5 - - 3 30 - - VL15 - - 6 - - 18 - - 7 - - 2 17 4 14 - - 26 - - VL18 - - 4 - - 11 7 - 13 - - 1 128 133 53 148 145 129 130 60 VL16VL19 - -- - 8 4 - - - -15 9 11 52 - - 13 11 - 7 - 1 1 5 26 1 - - - - -20 2 - 8 - - VL17VL21 - -- 4 6 1 - - - 227 2 46 - 10- 13 - - - 3 1 - - 3 5 - - 8 - - 203 30 3 - - VL18VL22 - -- 24 3- -- 511 2 7 - - 9 13 - - - 5 1 -128 - 133 - 53 6 148 - 145 21 129 3 130 - 60 - VL19VL23 - -- -4 -- -- -9 40 52 26 - 4 11 2 - - -1 -26 - - - 13- - - 152 8 - - - VL21VL29 - -4 11 3- -2 3 2 2 - -10 11 - - - 3 - - -3 -- -8 9 - -20 21 3 3 - - VL22VL30 - -2 -3 2- -5 3 2 2 - -9 5 - - - 5 ------6 9 - -21 11 3653 366- 365- VL32 - 2 - - 13 2 - 13 9 - - - - - 16 - 18 - - - VL23VLHG -- - -- 2- -- -40 - 26 - 224 212 ------13 - - -15 - 26- - - VL29 - 1 3 - 3 2 - 11 ------9 - 21 3 - - VL30 - - 2 - 3 2 - 5 ------9 - 11 365 366 365 VL32 - 2 - - 13 2 - 13 9 - - - - - 16 - 18 - - - VLHG - - 2 - - - - 22 21 ------26 - -

Remote Sens. 2018, 10, 1608 6 of 27

VLNDEF is the first Italian geodetic network in Antarctica [8] established in 1999, which is nowadays constituted by 27 sites distributed in a region which ranges over 500 km along North–South and 300 km in the East–West direction, including the “Mario Zucchelli Station” Italian base. It is a discontinuous network which was repeatedly surveyed during the Austral summer seasons. In 2008, three remote stations (VL01 Cape Hallett, VL05 Cape Philips, VL18 Starr Nunatak) were powered by a set of batteries and solar panels allowing them to acquire data for many months a year. Since the end of 2014, three VLNDEF markers (VL01, VL12, VL30) have been included in the POLENET permanent GNSS network. In this work, the whole VLNDEF dataset was reprocessed to include most recent surveys and improve the time series of site positions (observation files available at ftp://ftp.ira.inaf.it/pub/ogia/). Furthermore, 50 POLENET GNSS stations, and a subset of IGS stations were adopted according to criteria of homogeneous geographical distribution and quality of the GNSS time series. The gravimetric dataset is based on relative gravity surveys carried out in the Austral summer at 180 sites located over the bedrock, distributed in the VL within the PNRA activities. The absolute gravity value available for a gravimetric benchmark located near Mario Zucchelli Station was adopted as reference value [27]. For such relative gravity measurements, a Lacoste–Romberg model G-433 gravimeter with ZLS (Zero Length Spring) feedback was used. The accuracy of the terrestrial gravity measurements was better than 1 mGal in all cases. GNSS rapid static surveys were carried out to co-locate gravimetric sites in ITRF14 [28]. The aforementioned terrestrial gravimetric surveys play a major role in the creation of a reliable gravimetric map in NVL due to a previous lack of data within this region or to the poor spatial data distribution (Figure2). Terrestrial gravimetric observations were integrated with three different datasets: the 1980–1981 dataset of gravity profiles acquired by the German research institution (Bundesanstalt für Geowissenschaften und Rohstoffe—BGR) on the Antarctic Ocean (Explora Vessel), with successive integration with data provided by the OGS (Osservatorio Geofisico Sperimentale) [29]; the 1988–2005 gravity data acquired along profiles in the Ross Sea (Antarctica) in the framework of the OGS international research programs; and the 2015 airborne gravity data collected by British Antarctic Survey (BAS) [17], provided with the AntGG compilation [17] at an average distance of 10 km after the interpolation and an accuracy of 2–5 mGal. Although datasets were acquired at different epochs all data were merged to obtain a final VL gravimetric dataset without introducing significative errors. Gravimetric temporal variations are about 2 µGal/yr in Antarctica [30,31], which are too small in order to bias G-gravimeter relative errors that are about 1 mGal or worse.

3. Methodology

3.1. Global Navigation Satellite System (GNSS) Data Analysis Daily observations, acquired over the last 20 years, of 208 GNSS stations located around the world (Figure3; Supplementary Materials, Tables S1–S3) were processed using the Bernese GNSS software V5.2 [26]. The adopted double-difference approach enables the use of hydrostatic, wet Vienna Mapping Functions and provides a priori Zenith Hydrostatic Delay (ZHD; VMF1) [32]. The Zenith Total Delay (ZTD) estimation in Antarctica is a tough task that affects the accuracy of coordinate estimates. In order to reduce this effect, Bernese software was modified accordingly by adding the Global Pressure and Temperature 2 (GPT2) model [33], allowing in this way a more reliable results in the coordinate computation [34]. In order to study the displacements of VLNDEF sites within the Antarctic frame, the POLENET and IGS Antarctic permanent stations were analyzed along with the VLNDEF data. Given the availability of a long time series of observations, the whole dataset was completely reprocessed to obtain consistent and unbiased estimates of geodetic parameters [35–38], adding available data coming from new VLNDEF campaigns and permanent stations. Remote Sens. 2018, 10, 1608 7 of 27

To provide a complete picture of GNSS sites displacements with respect to an external reference frame, a number of global IGS stations was selected, fulfilling whenever possible the following conditions: (a) having homogeneous geographical distribution; (b) having a long-term continuous observation history; (c) having ITRF2014 coordinates and velocities; (d) being designated as IGS14 core network stations. The 20-year dataset was processed with the Bernese Processing Engine (BPE) tool. The analysis started from the observation files in the Receiver INdependent EXchange (RINEX) format, and aimed at producing and storing daily solutions in the Software INdependent EXchange (SINEX) format and normal equations (NEQs). A data sampling rate of 30 s and an elevation cutoff angle of 10◦ were used as the best compromise between the amount and the quality of data, this being a crucial aspect for high-latitude regions. The ionosphere-free linear combination (LC) of the L1 and L2 signals eliminates the first order ionospheric refraction, but also the Higher Order Ionosphere (HOI) and ray path bending correction terms were considered. The hydrostatic VMF1 and the wet mapping functions were used to compute ZTD. Corrections to a priori ZHD values were computed as piecewise linear functions with a time spacing of 1 h. An advanced ambiguity resolution scheme was included, using different resolution strategies that considered different baseline lengths. In detail, ambiguities were resolved in a baseline-by-baseline mode using the Melbourne–Wübbena combination (baseline < 6000 km), Quasi-Ionosphere-Free (QIF) approach (baseline < 2000 km), Phase-based widelane/narrowlane method (baseline < 200 km) and Direct L1/L2 method (baseline < 20 km). The a priori global ionosphere models (GIMs) provided by the Center for Orbit Determination in Europe (CODE) were used to support the QIF approach [26]. The whole dataset was organized into 14 clusters based on a regional criterion to reach a more suitable data analysis. Moreover, permanent and epoch-wise stations were assigned to different clusters. Products derived from the IGS repro2 campaign (ftp://cddis.gsfc.nasa.gov/gps/ products/repro2) and from official IGS solution, including IGS station coordinates, satellite orbits and Earth Orientation Parameters (EOP) were used together with the absolute phase-center corrections, applied for both satellite and receiver antennas. Corrections for Solid Earth, Ocean and Pole tides were applied following the International Earth Rotation and Reference Systems Service conventions (IERS2013); ocean-loading effects were taken into account using FES2014b [39] + TPXO.8 Atlas [40] models, including the CoM, (Center of Mass) correction for the Earth’s motion due to ocean tides. Table2 summarizes the complete list of parameters and models adopted.

Table 2. Parameters and models used in the GNSS data processing.

Solid Earth tide IERS Conventions Permanent tide Conventional tide free system: IERS Conventions Ocean Tides FES2004 (a) Pole Tides Linear trend for mean pole offsets: IERS Conventions FES2014b + TPXO8-Atlas including the CoM correction for the Ocean Loading motion of the Earth due to the ocean tides (b) Atmospheric Loading Not applied IGS weekly ERP files (X-pole. Y-Pole, UT1-UTC) used with IGS A priori information Precise orbits IG2 (c)/IGS (d) Subdaily EOP Model IERS2010 Nutation IAU2000R06 Hydrostatic delay Computed from 6-hourly ECMWF grids (e) Mapping functions VMF1 Wet delay Zero a priori model, 1 –h parameter estimated Gradients Zero a priori values, 24-h parameter estimated Phase center model igs14.atx (e) Radome Calibrations igs14.atx (e) Antenna height igs.snx (e) Horizontal offsets Applied A priori radiation pressure C061001 A priori ionosphere model CODE GIMs (f) (a) https://www.aviso.altimetry.fr/en/data/products/auxiliary-products/global-tide-fes/description-fes2004. html; (b) http://holt.oso.chalmers.se/loading/; (c) ftp://igs.ensg.ign.fr/pub/igs/products/repro2/; (d) ftp://ftp. igs.org/pub/product/; (e) http://ggosatm.hg.tuwien.ac.at/DELAY/; (f) ftp://ftp.aiub.unibe.ch/CODE/. Remote Sens. 2018, 10, 1608 8 of 27

The geodetic datum was defined by a No-Net Translation (NNT) condition, performing a minimum constraint solution during the NEQ stacking process, and fixing a sub-set of approximately 40 IGS stations, which were chosen outside Antarctica and having the most reliable and stable a priori ITRF2014 coordinates [28] (Figure3). The repeatability of the daily coordinates obtained after the least square adjustment process were of the order of few millimeters, despite the submillimeter formal errors provided by the software. Such daily solutions were used to derive the ground motion rates of the sites of the whole Antarctic network, estimating the linear trend of each GNSS station time series. This task was carried out using the Hector software, version 1.6 [41] by means of an automatic routine and default parameter setting. For continuous GNSS stations, offsets due to instrumental changes were considered both for the outlier detection/removal and for the linear trend estimate by preprocessing each time series [42]. In this procedure, the errors associated at each coordinate were not taken into account. Otherwise, for VLNDEF discontinuous GNSS station offsets were neglected. This criterion was also adopted for VL01, VL12, and VL30, although they are actually continuous GNSS stations, due to their short continuous time series. For all stations, the combination between the white noise model and the power-law noise model was adopted for the evaluation of the noise signal along the time series. The seasonal signal was also included in the estimation process of the linear trend. The final GNSS time series do not incorporate GIA corrections in the GNSS uplift model and, therefore, biases could affect the detected vertical displacements. However, previous paper focused on GIA modeling in the NVL highlighted very low vertical rates due to GIA effects. The displacements predict from the GIA models are furthermore characterized by low spatial variability [43]. Also far-field effects away from the areas of main melting phenomena have been modeled as tenths of mm/yr [44].

3.2. Gravimetric Data Analysis The terrestrial VL gravimetric data were acquired over the bedrock. As reference value [27], the absolute gravity value available for a gravimetric benchmark located near Mario Zucchelli Station was adopted. Gravimetric measurements were performed using a Lacoste-Romberg model G-433 gravimeter with ZLS feedback. The accuracy of the ground-based gravity measurements was better than 1 mGal in all cases. GNSS rapid static surveys were realized to co-locate the gravimetric sites within ITRF14 [28]. The reduction to the orthometric values was computed using the EGM2008 global geoid [45]. The ground gravimetric data were processed with a four steps procedure as follows:

(1) Earth and ocean tide corrections. These corrections are based on tidal parameter sets, or an ephemeris, that model the location of both the Moon and Sun celestial coordinates. The Solid Earth tide correction is a direct computation of the tidal potential [46]. The ocean tide correction is computed from loading parameters for the semidiurnal (M2, S2, N2, K2), diurnal (O1, P1, Q1, K1) and long-period (MF, Mm, Ssa) tidal harmonics [47]. The ocean-loading coefficients were provided from an external source, the free ocean provider (available at http://holt.oso. chalmers.se/loading/). Finally, the Solid Earth tide was removed from gravity measurements with a maximum correction of 0.5 µGal. (2) Pressure correction. Changes in atmospheric pressure imply variations in the mass of the air column above the gravity point of measurement. Thus, an increase (or decrease) in atmospheric pressure will cause a decrease (or increase) in the observed gravity. (3) Instrumental drift correction. Correction of gravimeter drift plays a significant role in the accuracy of all gravity surveys because it can reach up to 1 mGal/day. We calculated the instrument loop by a “loop approach”, which requires a base station as starting and final point of series of gravimetric measurements representing a single loop. For each loop, the base station was defined as the station with the longest time interval between repeated measurements. The drift effect was estimated by a least square fit of the weighted time series at this station with an nth order polynomial function. The weighting factor adopted was the inverse of the standard deviation associated with each Remote Sens. 2018, 10, 1608 9 of 27

measurement. In many cases, a first-order polynomial function was enough to remove short-term instrumental drift for loops with duration ranging from hours to a couple of days. (4) Network adjustment. Finally, data corresponding to series of relative gravity values were adjusted to a common reference point where absolute measurements collected near Mario Zucchelli station were provided [27].

The data were evaluated as FAG anomalies by means of [48]:

∆g0 = g − γ0 + δgF (1) where g is the observed gravity value; γ0 is the theoretical value according to the Geodetic Reference System 1967 (GRS 68); δgF is the FAG reduction. The BG anomaly was obtained from [49]:

∆g00 = ∆g0 + δgT + δgB (2) where δgT is the terrain reduction with an uniform density of 2.67 g/cm3 computed to Hayford Zone 02 on a spherical Earth (that is from 0 to 166.7 km); δgB is the reduction of the BG effect on a spherical Earth and to Hayford Zone 02. The formula used for this computation is that of Cassinis et al. [49]. The density used is also of 2.67 g/cm3 and the water density used is of 1.04 g/cm3. Therefore, it is necessary to be aware that the terrain effects in high mountain regions and steep coastal areas can easily reduce the accuracy by the order of 10 to 20 mGal. In order to evaluate the terrain reductions, we consider the Radarsat Antarctic Mapping Project Digital Elevation Model [50]. Since the data have been acquired over the bedrock, the terrain reductions were performed not considering the ice-sheet thickness. The absolute gravity values over terrestrial sites and the gravity anomalies computed are shown in Table S5 of the Supplementary Materials. After the aforementioned processing, the sparse terrestrial gravimetric data were combined with observations from airborne and shipborne gravity data by using a gridding procedure with a search radius of 10 km. Terrestrial measurements were considered as constraints in the interpolation procedure and the different accuracy of the gravimetric datasets was also considered. For this aim the Gravsoft package was used [51]. In the first step, the program GEOIP was used to compute the data gridding and downward continuing by least squares collocation using the planar logarithmic model for data with different data spacing. The same program was also used for interpolating functional values at the computation points from a grid using either a bilinear or a spline interpolation techniques. In order to use these interpolated values at the same points we used the bilinear technique. Finally, the data were merged by the Gravsoft GCOMB program [51]. It resulted in a gravimetric map with a grid spacing of 5 km. Comparing these results with ones obtained from ground gravimetric surveys, which were imposed as constraint (see Table S5 of the Supplementary Materials), for the FAG anomaly we obtained a maximum difference of 5 mGal with a standard deviation of 1 mGal, whereas for the BG anomaly the maximum difference was of 6 mGal with a standard deviation of 1.5 mGal. BG anomalies were used to model the crust thickness in the VL–Ross Sea Region. This was performed by a 3D modeling with prisms of 5 × 5 km wide using a dual density model (2800 kg/m3 for the crust and 3300 kg/m3 for the mantle) and equilibrated assuming a 34 km Moho depth for zero value anomaly [30,52]. The model was performed by a progressive Monte Carlo variation of prisms height driven by stochastic convergent steps with an in-house developed algorithm. Results present a root mean square (RMS) of 1.3 mGal and indicate an average difference of 5 km in the crustal thickness between VL and the Ross Sea. Remote Sens. 2018, 10, x FOR PEER REVIEW 10 of 27 Remote Sens. 2018, 10, 1608 10 of 27 4. Results 4. Results The processing resulted in maps of GNSS absolute velocities of sites that are represented in The processing resulted in maps of GNSS absolute velocities of sites that are represented in Figures 4–7Figures and 4listed–7 and in listed Table in 3 Table (see3 also(see alsoSuppleme Supplementaryntary Materials, Materials, Table Table S4). S4). In In particular, particular, Figures 4 and 5 provideFigures4 and maps5 provide of horizontal maps of horizontal displacements displacements at at regional regional and and continental continental scale. Similarly, scale. Similarly, Figures 6 andFigures 7 provide6 and7 provide a complete a complete picture picture of of verticalvertical displacements displacements detected detected on GNSS on sites. GNSS sites.

Figure 4.Figure Absolute 4. Absolute horizontal horizontal displacement displacement map map (mm/yr) (mm/yr) in Victoria in Victoria Land (± Landσ). (±σ).

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FigureFigure 5. Absolute5. Absolute horizontal horizontal displacement displacement map map (mm/yr)(mm/yr) at at continental continental scale scale (± (σ±).σ ).

TableTable 3. 3.Coordinates Coordinatesand and velocitiesvelocities of VLNDEF sites. sites.

Absolute Velocities (mm/yr) RelativeRelative Velocities Velocities (mm/yr) Absolute Velocities (mm/yr) (mm/yr) ID Lon. (°) Lat. (°) H (m) Ve ±σe Vn ±σn Vu ±σu Ve Vn Ven ±σen ◦ ◦ TNB1ID 164.10294 Lon. ( ) −74.69881 Lat. ( ) H72.2 (m) Ve 10 ±0.15σe Vn−12.1 ±0.16σn Vu 0.5 ± 0.39σu −Ve0.01 Vn−0.25 Ven0.25± σen0.22 VL01TNB1 169.72507 164.10294 −−72.4501474.69881 596.972.2 12.1 10 0.150.18 −−12.111.7 0.160.22 0.5−0.4 0.390.45 − 0.330.01 −−0.250.34 0.250.470.22 0.28 VL02VL01 167.37814 169.72507 −−72.5648872.45014 2047.2596.9 11.712.1 0.180.24 −−11.710.7 0.220.2 −−0.4 0.450.38 0.33 0.44 −0.34 0.82 0.47 0.930.28 0.31 VL03VL02 162.92641 167.37814 −−72.9505172.56488 2047.22469.6 11.7 11 0.24 0.23 −10.7−12 0.20.26 −−0.4 0.380.5 0.44 0.73 0.82−0.04 0.930.730.31 0.35 − − − − VL04VL03 169.74865 162.92641 −73.5182172.95051 2469.61834.6 11.8 11 0.230.18 −1211.9 0.260.18 −1.30.4 0.50.46 0.73 0.31 −0.040.59 0.730.670.35 0.25 VL04 169.74865 −73.51821 1834.6 11.8 0.18 −11.9 0.18 −1.3 0.46 0.31 −0.59 0.67 0.25 VL05VL05 169.61219 169.61219 −−73.0630773.06307 478.5 11.8 0.160.16 −−11.411.4 0.210.21 −−0.8 0.330.33 0.24 0.24 −−0.070.07 0.250.250.26 0.26 VL06VL06 164.69065 164.69065 −−74.3500074.35000 26712671 10.4 0.180.18 −12−12 0.170.17 0.80.8 0.550.55 0.180.18 −−0.170.17 0.250.250.25 0.25 VL07VL07 165.37930 165.37930 −−73.7599073.75990 2039.2 9.9 9.9 0.28 0.28 −−11.811.8 0.130.13 −−1.4 0.610.61 −−0.65 −−0.070.07 0.650.650.31 0.31 VL08VL08 163.73953 163.73953 −−73.7642873.76428 2655.4 10.5 0.150.15 −−11.911.9 0.370.37 0.9 0.9 0.38 0.38 0.27 0.27 −−0.030.03 0.270.270.40 0.40 VL09VL09 162.16939 162.16939 −−73.3307873.33078 2270.4 10.1 0.230.23 −−11.911.9 0.230.23 0.4 0.4 0.47 0.47 0.06 0.06 0.09 0.09 0.11 0.110.33 0.33 VL10 162.76859 −73.68846 2619.4 9.8 0.24 −11.9 0.26 −0.4 0.56 −0.22 0.13 0.26 0.35 VL10 162.76859 −73.68846 2619.4 9.8 0.24 −11.9 0.26 −0.4 0.56 −0.22 0.13 0.26 0.35 VL11 162.54167 −74.37143 2362.3 9.8 0.25 −11.5 0.14 −0.8 0.25 −0.03 0.5 0.50 0.29 VL11VL12 162.54167 163.72700 −−74.3714372.27444 2362.31933 10.9 9.8 0.12 0.25 −−12.811.5 0.250.14 0.5−0.8 0.420.25 − 0.210.03 −0.90.5 0.920.500.28 0.29 VL12VL13 163.72700 162.20497 −−72.2744474.84780 1460.31933 10.9 9.2 0.210.12 −−11.612.8 0.150.25 00.5 0.31 0.42 − 0.210.36 0.45−0.9 0.580.920.26 0.28 VL13VL14 162.20497 165.90570 −−74.8478073.22825 1460.32084 11.3 9.2 0.21 0.21 −−1211.6 0.250.15 −0.8 0 0.48 0.31 − 0.530.36 −0.270.45 0.590.580.33 0.26 VL14VL15 165.90570 163.71567 −−73.2282574.93426 −208428.1 11.3 9.4 0.260.21 −12−12 0.130.25 0.5−0.8 0.380.48 − 0.530.46 −−0.080.27 0.470.590.29 0.33 VL16 162.54549 −75.23256 311.3 10.1 0.23 −12.4 0.16 0.6 0.42 0.54 −0.43 0.69 0.28 VL15 163.71567 −74.93426 −28.1 9.4 0.26 −12 0.13 0.5 0.38 −0.46 −0.08 0.47 0.29 VL17 161.53874 −75.09513 683.5 8.8 0.27 −11.9 0.13 0.7 0.36 −0.54 0.2 0.58 0.30 VL16VL18 162.54549 162.59371 −−75.2325675.89853 311.358 10.1 9.1 0.150.23 −−11.912.4 0.20.16 10.6 0.35 0.42 − 0.540.18 0.05−0.43 0.190.690.25 0.28 VL17VL19 161.53874 161.78161 −−75.0951375.80497 809.8683.5 8.8 8.8 0.140.27 −−11.811.9 0.190.13 0.9 0.7 0.45 0.36 −−0.540.34 0.230.2 0.410.580.24 0.30 VL18VL21 162.59371 163.73293 −−75.8985371.66866 1899.458 8.4 9.1 0.520.15 −−11.711.9 0.220.2 1 1 0.15 0.35 −−0.182.49 0.230.05 2.500.190.56 0.25 VL19VL22 161.78161 162.04043 −−75.8049771.42187 274.9809.8 10.7 8.8 0.190.14 −−12.111.8 0.250.19 2.5 0.9 0.19 0.45 − 0.120.34 −0.040.23 0.130.410.31 0.24 − − − VL21VL23 163.73293 170.30467 −71.6686671.34582 1899.41119 12.6 8.4 0.21 0.52 −11.311.7 0.330.22 0.3 1 0.22 0.15 − 0.412.49 0.050.23 0.412.500.39 0.56 VL29 163.89628 −71.15408 1624.4 11.3 0.05 −11.9 0.29 2.8 0.53 0.25 −0.01 0.25 0.29 VL22VL30 162.04043 162.52514 −−71.4218770.59872 1491.5274.9 10.710.6 0.240.19 −−12.612.1 0.230.25 1.4 2.5 0.46 0.19 − 0.120.31 −−0.590.04 0.670.130.33 0.31 VL23VL32 170.30467 166.16457 −−71.3458271.73310 17841119 12.612.2 0.30.21 −−12.511.3 0.370.33 0.7 0.3 0.31 0.22 0.88 0.41 −−0.860.05 1.230.410.48 0.39 VL29VLHG 163.89628 162.20172 −−71.1540875.39797 1624.4165.6 11.3 9.2 0.170.05 −−12.211.9 0.110.29 0.5 2.8 0.28 0.53 − 0.250.24 −−0.160.01 0.290.250.20 0.29

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VL30 162.52514 −70.59872 1491.5 10.6 0.24 −12.6 0.23 1.4 0.46 −0.31 −0.59 0.67 0.33

VL32 166.16457Remote Sens. 2018−71.73310, 10, 1608 1784 12.2 0.3 −12.5 0.37 0.7 0.31 0.88 −0.8612 of 271.23 0.48 VLHG 162.20172 −75.39797 165.6 9.2 0.17 −12.2 0.11 0.5 0.28 −0.24 −0.16 0.29 0.20

Figure 6. Vertical rates (mm/yr) detected in Victoria Land. Red dots denote uplifting sites, blue dots Figure 6. Vertical rates (mm/yr) detected in Victoria Land. Red dots denote uplifting sites, blue dots denote subsidingdenote subsiding sites. Gray sites. dots Gray show dots show not notsignificant significant movements movements followingfollowing the the±σ ±criteria.σ criteria. LTF: Lillie–TuckerLTF: Lillie–TuckerFault; RAF: Fault; Rennick–Aviator RAF: Rennick–Aviator Fault. Fault.

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Figure 7. 7. VerticalVertical rates rates (mm/yr) (mm/yr) detected detected at continental at continental scale. Red scale. dots Reddenote dots uplifting denote sites, uplifting blue sites,dots denote blue subsiding dots denote sites, subsiding black stars sites,represent black the starsout of representrange displayed the out values of (BENN range displayed10.8 ± 1.46 valuesmm/yr, (BENNBERP 28.9 10.8 ± 2.3± mm/yr,1.46 mm/yr, DUPT 12.0 BERP ± 0.74 28.9 mm/yr,± 2.3 FONP mm/yr, 13.9 ± DUPT0.77 mm/yr, 12.0 ±SPGT0.74 10.2 mm/yr, ± 1.29 FONPmm/yr, 13.9TOMO± 55.40.77 ± mm/yr,3.58 mm/yr; SPGT Supplementary 10.2 ± 1.29 Ma mm/yr,terials, Table TOMO S4); 55.4 gray± dots3.58 show mm/yr; not Supplementarysignificant movements Materials, (±σ Table). S4); gray dots show not significant movements (±σ).

To findfind residualresidual displacements,displacements, thethe contribution of displacements due to the global plate circuit was removedremoved byby computingcomputing the the Euler Euler pole pole location. location. For For the the aim, aim, a subseta subset of of 95 95 GNSS GNSS stations stations located located on theon the Antarctic Antarctic plate plate was was selected selected (see (see Supplementary Supplementary Materials, Materials, Table Table S3), S3), and and used used to compute to compute the Eulerthe Euler pole pole parameters parameters (Table (Table4) by 4) means by means of the of Euler the Euler Pole Pole Calculator Calculator (EPC) (EPC) software software [ 53]. [53]. North North and Eastand East absolute absolute velocities velocities were were adopted adopted as input,as input, along along with with their their uncertainties uncertainties and and coordinates. coordinates. A weighted least squares inversion provided the Euler pole coordinates to minimize the residual horizontal velocities.

Table 4. Euler poles rotational coordinates (“mas yr−1” = milliarcsecond/yr; Table 4. Euler poles rotational coordinates (“mas yr−1” = milliarcsecond/yr; “° Myr−1” = “◦ Myr−1” = degree/Millions yr). degree/Millions yr). Model NS (a) ω (mas yr−1) ω (mas yr−1) ω (mas yr−1) ω (◦ Myr−1) x ωx y ωy ωz z ω Model NS (a) −0.260(mas yr−1) (mas−0.325 yr−1) (mas yr0.638−1) (° Myr−1) 0.212 VLNDEF_2018 95 ± ± ± ± 0.005 −0.260 −0.0040.325 0.6380.016 0.212 0.004 VLNDEF_2018 95−0.248 −0.324 0.675 0.219 ITRF14 (b) 7 ±0.004 ±0.005 ±±0.0040.004 ±0.016±0.008 ±0.004 ±0.002 −0.248 −0.324 0.675 0.219 ITRF14 (b) 7 (a) Number of sites; (b) from [54]. ±0.004 ±0.004 ±0.008 ±0.002 (a) Number of sites; (b) from [54].

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SubtractingSubtracting the rigid the rigidplate plate moveme movementnt from from absolute GNSS GNSS site site velocities, velocities, relative relative movements movements among stationsamong stationswere illustrated were illustrated (Figures (Figures 88 and and 99;; Table Table3). 3).

Figure Figure8. Residual 8. Residual horizontal horizontal displacement displacement map map (mm/yr) (mm/yr) in Victoria in Victoria Land ( ±Landσ). (±σ).

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FigureFigure 9. Residual 9. Residual horizontal horizontal displacement displacement map map (mm/yr) (mm/yr) at continentalat continental scale (scale±σ). (±σ).

The estimated Euler pole location was found very close to that provided within the definition of The estimated Euler pole location was found very close to that provided within the definition of the ITRF2014 [54]. This computation (see VLNDEF_2018 model in Table4) constrains the motion of the ITRF2014the Antarctic [54]. This plate computation well and minimizes (see theVLNDEF_2018 relative velocities. model in Table 4) constrains the motion of the AntarcticThe plate results well obtained and minimizes from the processingthe relative of velocities. gravimetric data are depicted in Figures 10–13. TheIn results particular, obtained Figures 10from and the11 show processing the BG anomalies, of gravimetric whereas data Figures are 12 depictedand 13 represent in Figures the FAG 10–13. In particular,anomalies Figures at 10 regional and 11 and show continental the BG scales. anomalies, whereas Figures 12 and 13 represent the FAG anomalies atIn regional this study, and the continental tectonic characteristics scales. of VL were proposed reading both geodetic movements and gravity station measurements. Analyses were conducted at regional and continental scales. The regional extent relies with the geodetic and gravimetric observation within the VL, whereas the continental scale involves all data collected over the Antarctic plate. Absolute and relative three-dimensional displacement rates of all GNSS sites located in Antarctica were obtained by geodetic processing. The main absolute motion of VL is towards SE (Table3; Figure4; Ve 9.9 ± 0.26 mm/yr, Vn −11.9 ± 0.27 mm/yr).

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Figure 10. FigureGridded 10. Gridded data of data Bouguer of Bouguer Gravity (BG) (BG) anomalies anomalies (mGal) in (mGal) Victoria Landin Victoria (contour interval Land (contour at 50 mGal). interval at 50 mGal). The velocity differences between East and West Antarctica (Figure5) relate to the closeness of the rotation pole to East Antarctica, and are confirmed by the separated pole computation performed using the velocities of the two regions. These two poles show an angular difference of less than 2◦. The predicted relative velocity at the boundary between East and West Antarctica is less than 0.5 mm/yr and confirms that the relative movements between the two regions are negligible. This coherent

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movement is also confirmed by the residual velocity vectors (Figure8) that, in the most cases, are within the error range. Exceptions are detected in the NVL between the Rennick–Aviator and Lillie–Tucker faults. This sector includes several VLNDEF sites: VL02, VL03, VL12, VL14, VL21 and VL32, which exhibit residual horizontal velocities up to 2.5 mm/yr (VL21). The presence of scattered residual velocities highlight relative movements as signs of possible active tectonics in this sector. This tectonic pattern is related to the on-land propagation of the Tasman Fracture Zones in NVL (Figure1)[ 23]. The uplift rate field shows a general raising in VL with velocities up to 2.5 mm/yr. This general trend does not occur in the sector of NVL between 73◦ and 74◦S, where there is a slight subsidence (less than 1 mm/yr). This sector and the highest uplift rates are again located in the active tectonic region of NVL, Remote Sens. 2018as clearly, 10, x shownFOR PEER by the REVIEW residual velocity analysis, and it may represent the effects of tectonic activity. 17 of 27

Figure 11. GriddedFigure 11. Gridded data of data BG of anomalies BG anomalies (mGal) (mGal) at continentalcontinental scale scale (contour (contour interval interval at 50 mGal). at 50 mGal).

The spatial distribution of absolute vertical velocities may relate to the activity of the known In thisregional study, NW-SE the faultstectonic [9,15, 40characteristics]. The comparison of of suchVL distributionwere proposed with the modelsreading does both not geodetic movementssubstantially and gravity change station this interpretation measurements. [14], except Analyses for the highwere uplift conducted rates in the at Antarctic regional Peninsula. and continental scales. The regional extent relies with the geodetic and gravimetric observation within the VL, whereas the continental scale involves all data collected over the Antarctic plate. Absolute and relative three-dimensional displacement rates of all GNSS sites located in Antarctica were obtained by geodetic processing. The main absolute motion of VL is towards SE (Table 3; Figure 4; Ve 9.9 ± 0.26 mm/yr, Vn −11.9 ± 0.27 mm/yr). The velocity differences between East and West Antarctica (Figure 5) relate to the closeness of the rotation pole to East Antarctica, and are confirmed by the separated pole computation performed using the velocities of the two regions. These two poles show an angular difference of less than 2°. The predicted relative velocity at the boundary between East and West Antarctica is less than 0.5 mm/yr and confirms that the relative movements between the two regions are negligible. This coherent movement is also confirmed by the residual velocity vectors (Figure 8) that, in the most cases, are within the error range. Exceptions are detected in the NVL between the Rennick– Aviator and Lillie–Tucker faults. This sector includes several VLNDEF sites: VL02, VL03, VL12, VL14, VL21 and VL32, which exhibit residual horizontal velocities up to 2.5 mm/yr (VL21). The presence of scattered residual velocities highlight relative movements as signs of possible active tectonics in this sector. This tectonic pattern is related to the on-land propagation of the Tasman Fracture Zones in NVL (Figure 1) [23]. The uplift rate field shows a general raising in VL with velocities up to 2.5 mm/yr. This general trend does not occur in the sector of NVL between 73° and 74°S, where there is a slight subsidence (less than 1 mm/yr). This sector and the highest uplift rates are again located in the active tectonic region of NVL, as clearly shown by the residual velocity analysis, and it may represent the effects of tectonic activity.

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The spatial distribution of absolute vertical velocities may relate to the activity of the known regional NW-SE faults [9,15,40]. The comparison of such distribution with the models does not substantially change this interpretation [14], except for the high uplift rates in the Antarctic Peninsula.Remote Sens. 2018, 10, 1608 18 of 27 The FA anomaly reflects the short wavelength components of the Earth’s gravity field induced by topography.The On FA one anomaly hand, reflects the theFAG short tends wavelength to follow components the topographical of the Earth’s gravityhighs and field induceddepressions. On the other byhand, topography. the BG On anomalies one hand, theare FAG influenced tends to follow by the the topographicallateral change highs of and crustal depressions. thickness that produces densityOn the other variations hand, the [55]. BG anomalies The comparison are influenced between by the lateralthese changetwo gravity of crustal anomalies thickness that allows us to produces density variations [55]. The comparison between these two gravity anomalies allows us to infer considerationsinfer considerations on the on geot theectonics geotectonics of ofthe the studied studied region. region.

Figure 12.Figure Gridded 12. Gridded data dataof Free-Air of Free-Air GravityGravity (FAG) (FAG) anomalies anomalies (mGal) in (mGal) Victoria Landin Victoria (contour intervalLand (contour at 40 mGal). interval at 40 mGal).

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Figure Figure13. Gridded 13. Gridded data dataof FAG of FAG anomalies anomalies (mGal) (mGal) at continentalcontinental scale scale (contour (contour interval interval at 40 mGal). at 40 mGal). 4.1. Free Air Gravity (FAG) Anomaly Interpretation 4.1. Free Air Gravity (FAG) Anomaly Interpretation NVL is characterized by the irregular spatial distribution of the FAG anomaly which includes NVLthe is highest characterized and lowest by values. the irregular FAG anomalies spatial indistribution the Ross Sea of are the close FAG to anomaly zero and which assume includes a the highestN–S trendand lowest as in the values. Terror Rift,FAG Victoria anomalies Land in Basin the Ross and Central Sea are Trough close (Figureto zero 11 and). The assume TAM a N–S (Trans Antarctic Mountains) is associated with positive values and is interrupted by a very strong trend as in the Terror Rift, Victoria Land Basin and Central Trough (Figure 11). The TAM (Trans negative minimum value (about −200 mGal) at the 75◦S parallel. A second interruption, again with a Antarcticsimilar Mountains) strong negative is associated minimum with value, positive is present va atlues the 79and◦S, is south interrupted of McMurdo by Sound.a very Mt. strong Erebus negative minimumand Mt.value Melbourne, (about − the200 active mGal) volcanoes at the in75°S this parallel. region, have A associatedsecond interruption, positive maxima. again The with Rennick a similar strong Geodynamicnegative minimum Belt is characterized value, is present by an abrupt at the step 79°S, from sout−200h of mGal McMurdo (to the West) Sound. to +160Mt. mGalErebus and Mt. Melbourne,(on the NE the side). active Its influence volcanoes on the in topography this region, is evident have inassociated the FAG anomaly positive map maxima. that suggests The itsRennick GeodynamicSE continuation Belt is characterized to the Lanterman by Range.an abrupt Along step the from Lillie–Tucker −200 mGal fault another(to the negativeWest) to minimum +160 mGal (on − − the NEzone side). is presentIts influence in its NW on sectorthe topography ( 150 mGal) is whereas evident in thein the SE sectorFAG aanomaly step from map100 that mGal suggests (SW) to its SE +200 mGal (NE) is present. The latter is due to the Admiralty Mts. continuation to the Lanterman Range. Along the Lillie–Tucker fault another negative minimum zone At the continental scale, FAG anomaly of Antarctica evidences the main peaks. Together with VL is presentand in the its TAM, NW Gambursev sector (−150 Mts, mGal) Antarctica whereas Peninsula, in the Dronning SE sector Maud a step Land, from mountains −100 mGal of Enderby, (SW) to +200 mGal (NE)Mac Robertson,is present. and The Queen latter Maryis due lands to the can Admiralty be tracked. Mts. In FAG anomaly of Antarctica (Figure 12) Atthe the continuation continental of scale, TAM FAG to the anomaly South is well of Antarc visibletica and evidences is interrupted the by main a minimum peaks. negativeTogether with VL and the TAM, Gambursev Mts, Antarctica Peninsula, Dronning Maud Land, mountains of Enderby, Mac Robertson, and Queen Mary lands can be tracked. In FAG anomaly of Antarctica (Figure 12) the continuation of TAM to the South is well visible and is interrupted by a minimum negative zone around about the 81°S parallel. Four regional main depressions, the Ross and Weddel Sea, the Wilkes and Aurora Basin, are evident in the map as zones with negative values (from about −80 to −40 mGal). A series of narrow sharp negative minima characterize both East and West Antarctica. In West Antarctica is present a narrow depression immediately to the East of the Ellsworth Mts, while the nearby Bentley Trench does not seem to show a significant correlation with the FAG anomaly. In East Antarctica, several sharp negative minima are present to the West of

Remote Sens. 2018, 10, 1608 20 of 27 zone around about the 81◦S parallel. Four regional main depressions, the Ross and Weddel Sea, the Wilkes and Aurora Basin, are evident in the map as zones with negative values (from about −80 to −40 mGal). A series of narrow sharp negative minima characterize both East and West Antarctica. In West Antarctica is present a narrow depression immediately to the East of the Ellsworth Mts, while the nearby Bentley Trench does not seem to show a significant correlation with the FAG anomaly. In East Antarctica, several sharp negative minima are present to the West of Pensacola Mts, in the Coats Land, in the Lambert Rift and its inland propagation towards the foothill of the Gamburtsev Mts, in the Vostok Lake, in the northern Queen Mary Land, in the Adventure Trench and W to Mawson Peninsula. A further negative zone girdles the northern part of Dronning Maud Land.

4.2. Bouguer Gravity (BG) Anomaly Interpretation The BG anomaly represents the gravitational action because of the material existing below the considered surface. A large mountain chain causes the lithosphere deflection as consequence of the phenomenon of Airy’s isostatic compensation. Since the Bouguer correction does not take into account the roots of the mountains, BG anomalies above mountains are negative and above oceans are positive. The mantle rocks, which are more dense than the crustal ones, are at a lower depth in oceanic areas and at greater depths under the continental mountains. This generates, respectively, relative excesses and gravity defects, which in Bouguer’s maps are characteristic of isostatic conditions. BG anomaly map gives a clear idea of subsurface density. Negative values imply lower density beneath the measurement point, while the positive ones indicate higher density. Positive anomalies generally indicate the presence of thin crust (about 20 km), which is evident where the Moho rises. Offshore, FAG anomalies are comparable in size and shape with BG anomalies mainly due to shallow-water and uniform bathymetry. Negative BG anomalies with a large wavelength under the mountains are related to well-defined crustal roots and indicate the presence of lithosphere subduction [55]. The BG anomaly map (Figure9) evidences the presence of two regions: the Ross Sea is characterized by positive values while negative values are present in the VL. The BG anomaly in the Ross Sea shows rather homogeneous values with relative maxima corresponding to the Victoria Land Basin and Central Trough depressions, similarly to the FAG anomaly map (Figure 11). The narrow Terror Rift does not show a marked signature, and it can refer to the local behavior of the lithosphere that elastically sustains it, thus preventing its isostatic counterbalance. In VL, the BG anomaly shows an East–West gradient with decreasing negative values towards the Ross Sea, as expected for the rift shoulder of an extensional basin (10 Brink). In correspondence of the Bowers Mts, BG anomalies show an abrupt change of over 100 mGal as highlighted by Ferraccioli [56]. This change may relate to the presence of a step in the Moho depth along the Rennick Geodynamic Belt (Figure1). On the other hand, the apparent regional lack of such a step at regional scale points to the strike-slip role of this geodynamic belt. The negative values associated with Mt. Erebus are an indication of the presence of its root (local crustal thickening), and are possibly due to the presence of intrusions or alternatively to flexural roots (e.g., Aitken [57]). This evidence contrasts with the absence of anomaly at the other active volcano, Mt. Melbourne. The irregular spatial distribution of the anomaly values, which characterize NVL, may be easily related to local transpression/transtension due to the activity of the strike-slip tectonics produced along the propagation of the Balleny and Tasman fracture zones in Cenozoic times [23]. This strike-slip tectonic regime in NVL is, therefore, responsible for the presence of the scattered anomalies with a radius of the order of few tens of kilometers. This contrasts with the extensional regime along the VL coastline that is clearly depicted by the transition band between positive and negative BG anomalies. At continental scale, the BG anomaly of Antarctica confirms the dual distribution between East and West Antarctica, with positive values in the latter and negative values in the former. In West Antarctica, negative values correspond to the roots (i.e., larger crustal thickness), at the Antarctic Peninsula, of the Ellsworth Highlands, and of the Mary Byrd Land. In East Antarctica wide negative anomalies Remote Sens. 2018, 10, 1608 21 of 27 are related to the roots of the TAM and of the main subglacial mountains, the Gamburtsev Mts and Dronning Maud Land. Relative negative minima characterize the Resolution Highland. A wide zone of negative values is present at the highlands between the Gamburtsev Mts and the Dronning Maud Land. The positive anomaly in the Lambert rift confirms the crustal thinning of this region. The large positive values that surround most of the Antarctic coastline represent the effect of the combined thinning of the crust towards the ocean and the border effect of the gridding. The moderate negative values in the Wilkes and Aurora basins may reflect the presence of a buried sedimentary cover as proposed by Scheinert et al. [17] and Jordan et al. [25].

5. Discussion Gravity anomalies identify two main sectors in NVL. The first is a NW region, which is characterized by small-sized BG anomalies with wavelengths of the order of some tens of kilometers. In this sector, the relative minimum values (differences up to 200 mGal) are sharper than the maximum values (100 mGal), which are associated with high uplift rates (Figure5). The second sector is a SE region, which shows low BG anomaly values that correspond to general subsiding velocities. The lowest Bouguer values correspond to the roots of the highest elevations (Mt. Murchison) of the region. Absolute plate velocities between the 140◦E and 40◦W, including VL and West Antarctica with the Peninsula, are similar in their magnitude, around 10 mm/yr, and direction, roughly around the 135◦E meridian. This contrasts with absolute velocities of stations located in the rest of East Antarctica, which are characterized by lower magnitudes and more variable directions. This last point relates to their closer position to the rotation pole. The comparison between these velocities and the spreading of oceans surrounding Antarctica shows a quite similar trend (Figure 14). The Southern Ocean expansion between 90◦E and 140◦E is nearly perpendicular to the East Antarctica continental margin and the ridge zone is characterized by few transforms with small offsets. The spreading style of the Southern Ocean changes abruptly between 140◦E and 180◦E meridians, being characterized by a series of transforms, the Tasman and Balleny fracture zones, that crosscut the whole ocean and propagate on land both in Antarctica [23,24] and Australia [58] with a right lateral motion. As a consequence, the spreading of the Southern Ocean becomes nearly parallel to the NVL coastline. The pathway of these fracture zones is nearly parallel to the absolute velocity vectors from VL to the Antarctic Peninsula, which represents the only convergent margin of the Antarctic continent. The pathway of the Balleny and Tasman Fracture Zones is nearly parallel to the absolute velocity vectors from VL to the Antarctic Peninsula (Figure 14), and represents the only convergent margin of the Antarctic continent. This suggests that the activity along these fracture zones is associated with the observed rotation of the Antarctic continent. This relation still leaves the question about the leading mechanism for the clockwise rotation of Antarctica open, i.e., being the effect of differential ocean spreading or due to an active continent rotation accommodated by the ocean spreading [59]. The presence of active right-lateral offset along the onshore prosecution of these fracture zone into NVL [21,23,24] suggests that the former hypothesis better describes the present-day geodynamic environment of the Antarctic continent and the engine reasonably resides in the mantle flow associated with the ocean spreading. The presence of the Cenozoic deformation belt in East Antarctica Craton [22,60] characterized by the presence of a subglacial lake cluster [61,62], could well represent the effect of this ocean-driven rotation. The activity role of the Tasman and Balleny fracture zones is limited along their zone of influence as in the NVL and the absolute vertical velocities confirm this framework. The general uplift in the Antarctic Peninsula and in the eastern part of West Antarctica is characterized by locations with movements exceeding 10 mm/yr and up to over 50 mm/yr (see in Figure7 the stations indicated with a black star). These values are larger than ones predicted by the GIA models [63,64] and may easily relate to the active tectonics, including the presence of converging margins. The tilting in NVL is the Remote Sens. 2018, 10, 1608 22 of 27 effect of the activity of the on-land southern propagation of the fracture zones. In the other regions of East Antarctica, closer to the plate rotation pole and characterized by a direction of ocean spreading perpendicular to the continent margins, there are not significant vertical movements. Remote Sens. 2018, 10, x FOR PEER REVIEW 22 of 27

Figure 14. Absolute velocities of the Antarctica continent within the frame of the Southern Figure 14. Absolute velocities of the Antarctica continent within the frame of the Southern Hemisphere Hemisphere plate tectonics focused in the last 70 Ma (orthographic projection, ocean floor age data plate tectonics focused in the last 70 Ma (orthographic projection, ocean floor age data from [59]). from [59]). The clockwise rotation of Antarctica is associated with the ocean expansion nearly always The clockwise rotation of Antarctica is associated with the ocean expansion nearly always at high at high angle with the continent margins. This contrast with the geodynamic in the Victoria Land, angle with the continent margins. This contrast with the geodynamic in the Victoria Land, where, where, related to the eccentric position of the rotation pole with respect to Antarctica, the Tasman related to the eccentric position of the rotation pole with respect to Antarctica, the Tasman and Balleny and Balleny fracture zones have entered within the continent with an extra right-lateral displacement fracture zones have entered within the continent with an extra right-lateral displacement since the since the Upper Cenozoic [21,23,24]. Upper Cenozoic [21,23,24].

6. Conclusions From the analysis of the observed GNSS data collected in a time span of about 20 years, improvement in the velocity uncertainty was obtained together with a new global framework of the Antarctic geodynamic behavior. New detailed gravimetric maps were elaborated merging original terrestrial datasets with airborne and shipborne data. The updated geodetic and gravimetric maps’ interpretation permits us to highlight the geodynamic characteristics of Victoria Land (VL) in the frame of the Antarctic plate motion.

Remote Sens. 2018, 10, 1608 23 of 27

6. Conclusions From the analysis of the observed GNSS data collected in a time span of about 20 years, improvement in the velocity uncertainty was obtained together with a new global framework of the Antarctic geodynamic behavior. New detailed gravimetric maps were elaborated merging original terrestrial datasets with airborne and shipborne data. The updated geodetic and gravimetric maps’ interpretation permits us to highlight the geodynamic characteristics of Victoria Land (VL) in the frame of the Antarctic plate motion. The Victoria Land Network for Deformation control (VLNDEF) processing highlights a movement of VL towards the SE (Ve 9.9 ± 0.26 mm/yr, Vn −11.9 ± 0.27 mm/yr) with a pathway nearly parallel to the transforms, which start from the Tasman and Balleny fracture zones and propagate on land in Antarctica. The velocity difference between East and West Antarctica is due to the close position of the rotation pole to East Antarctica while the similarity between VLNDEF_2018 and ITRF14 poles shows that ITRF14 system is well defined in Antarctica. BG anomalies confirm the differences between the thickness of the Earth’s crust of East and West Antarctica, while the relative velocities of the Antarctic stations demonstrate that the movements between the two regions are negligible. Some exceptions are due to stations that show active regional tectonics. NVL data confirm the highest uplift rates, while in the sector between 73◦S and 74◦S significant subsidence due to the right-lateral offset along the on land reactivated NNW–SSE regional faults was detected. The BG anomaly maps show the thinning of the crustal thickness from VL toward the Ross Sea. The NW region, characterized by small BG anomaly values, is associated with high uplift rates while the SE region presents low BG anomaly values and corresponding subsidence phenomenon. Unreported density variations produce the lowest BG values in the highest elevations (Mt. Murchison) of the region whereas gravity anomalies are mostly related to vertical movements and evidence the effect of dip-slip tectonics. The GNSS data reveal present day activity of the Rennick Geodynamic Belt, while gravity data is interpreted to support dominantly strike slip movement in this region over the Cenozoic. Tectonic dislocations are detected in the gravity anomaly map along the Rennick Geodynamic Belt, suggesting its southern continuation south of the Lanterman Range. The revealed scattered gravimetric anomalies confirm the differences between the modeled GIA values and the observed uplift values that can be related to deep-seated regional scale structures.

Supplementary Materials: The following are available online at http://www.mdpi.com/2072-4292/10/10/ 1608/s1, Table S1: Time span of the processed GNSS stations. The GNSS dataset consists of VLNDEF (ftp://ftp.ira.inaf.it/pub/ogia/), POLENET (ftp://data-out.unavco.org/pub/rinex/obs/) and IGS (ftp://igs.ensg.ign.fr/pub/igs/data/) stations. In red the stations located in Antarctica, Table S2: List of the processed GNSS stations (4-character codes) and their time span considered. A star (*) before the file name indicate span less than one year while a percentage (%) shows span less than two years but more than one year. The stations marked with a star (*) or with a percentage (%) were not considered reliable due the short time series which were reflected in large uncertainties, Table S3: Antarctic continuous GNSS sites used in this paper. For each year the number of daily observations is given. The last column indicate the accumulated total amount of daily observations, Table S4: The columns display the approximate geographic coordinates and the velocities of the Antarctic stations, Table S5: The columns display the approximate geographic coordinates of the terrestrial points located in VL, the gravity values observed and the gravity anomalies computed. H = orthometric height (Earth Gravitational Model 2008—EGM2008 [43]); BGA = Bouguer Gravity anomaly; FAGA = Free-Air Gravity anomaly; the deviation of the merged gravimetric surface on the fiducial points. Author Contributions: F.M., M.N., L.V. and A.Z. conceived and designed the paper; A.C., M.D., P.C., F.M., F.S., P.S., L.V. and A.Z. joined in Antarctic Scientific Expeditions; P.S. analyzed the geophysical data; P.C. and F.S. developed the geodynamic and tectonic interpretation of the data results partly within the PNRA, G-IDEA project; L.M., M.N. and A.Z. analyzed the GNSS data. A.C. is the Principal Investigator of geodetic projects supported by PNRA, Co-Chair with Mirko Scheinert (Technische Universität, Dresden, Germany) of the Geodetic Infrastructure of Antarctica (GIANT) Expert Group within the Scientific Committee on Antarctic Research (SCAR). All authors contributed to the writing of the paper. Funding: Part of this work was supported by the Italian National Program for Antarctic Research (PNRA). Acknowledgments: Several researchers contributed to the field activities. The figures were produced with the Generic Mapping Tool (GMT) software [65]. Remote Sens. 2018, 10, 1608 24 of 27

Conflicts of Interest: The authors declare no conflict of interest.

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